Yuji Orihashi

Reduction of mercury background on ICP-mass spectrometry for in situ U-Pb age determinations of zircon samples

Precise 238U–206Pb ages for three Phanerozoic zircons ( 300 mL) resulted in lowering the signal intensity of Pb and U. A series of measurements for Hg signal intensity and elemental sensitivity of the instrument revealed that the highest S/N ratio (minimal Hg signal with highest signal intensity of analytes) was obtained when 200 mL of activated charcoal was used. By filtering the Ar make-up gas using the charcoal filter, the signal intensity of 204Hg could be reduced down to <100 s−1, which is almost an order of magnitude lower than that observed without the Hg-trap. With the charcoal filter device, the uncertainty on the measured 204Pb intensity could be reduced remarkably, and this resulted in improvements in reliability of the 206Pb and 207Pb intensity data after the correction of non-radiogenic Pb (common-Pb). This is especially important to improve the accuracy and precision of the U–Pb age data for young zircons. We have measured U–Pb ages of three Phanerozoic zircons (≲600 Ma) from 18–20 μm pit sizes. The resulting 238U–206Pb ages for zircons from Sri Lanka (SL13) and Fish Canyon Tuff (FCT) were 566 ± 26 Ma (2SD, n = 10) and 27.6 ± 1.8 Ma (2SD, n = 13), respectively, showing excellent agreement with the previously reported values. The 238U–206Pb age for Mount Dromedary Igneous Complex (108.8 ± 3.9 Ma) was significantly older than the widely accepted value (98.7 ± 0.6 Ma). The possible cause of this discrepancy will be discussed in the text. The data obtained here demonstrate that the dating of Phanerozoic zircons from 20 μm ablation pits using LA-ICPMS is a real possibility.

Discovery of New Hydrothermal Activity and Chemosynthetic Fauna on the Central Indian Ridge at 18°–20°S

Indian Ocean hydrothermal vents are believed to represent a novel biogeographic province, and are host to many novel genera and families of animals, potentially indigenous to Indian Ocean hydrothermal systems. In particular, since its discovery in 2001, much attention has been paid to a so-called ‘scaly-foot’ gastropod because of its unique iron-sulfide-coated dermal sclerites and the chemosynthetic symbioses in its various tissues. Despite increasing interest in the faunal assemblages at Indian Ocean hydrothermal vents, only two hydrothermal vent fields have been investigated in the Indian Ocean. Here we report two newly discovered hydrothermal vent fields, the Dodo and Solitaire fields, which are located in the Central Indian Ridge (CIR) segments 16 and 15, respectively. Chemosynthetic faunal communities at the Dodo field are emaciated in size and composition. In contrast, at the Solitaire field, we observed faunal communities that potentially contained almost all genera found at CIR hydrothermal environments to date, and even identified previously unreported taxa. Moreover, a new morphotype of ‘scaly-foot’ gastropod has been found at the Solitaire field. The newly discovered ‘scaly-foot’ gastropod has similar morphological and anatomical features to the previously reported type that inhabits the Kairei field, and both types of ‘scaly-foot’ gastropods genetically belong to the same species according to analyses of their COI gene and nuclear SSU rRNA gene sequences. However, the new morphotype completely lacks an iron-sulfide coating on the sclerites, which had been believed to be a novel feature restricted to ‘scaly-foot’ gastropods. Our new findings at the two newly discovered hydrothermal vent sites provide important insights into the biodiversity and biogeography of vent-endemic ecosystems in the Indian Ocean.

Petrology of a mantle-derived rhyolite, Hokkaido, Japan

Abstract The Miocene Kitami rhyolite, consisting of orthopyroxene and plagioclase-phyric lavas and dikes, occurs on the back-arc side of the Kuril arc with coeval basalts and Fe-rich andesites. Temperatures estimated from orthopyroxene–ilmenite pairs exceed 900°C. Although the whole rock compositions of the Kitami rhyolite correspond to S-type granites (i.e., high K, Al, large ion lithophile elements, and low Ca and Sr), Sr–Nd isotope compositions are remarkably primitive, and similar to those of the coeval basalts and andesites. They are distinct from those of lower crustal metamorphic rocks exposed in the area. Comparison of chondrite-normalized rare earth element (REE) patterns between the rhyolite and the basalts and andesites show that the rhyolite is more light REE enriched, but has similar heavy REE contents than the basalts. All rhyolites show negative Eu anomalies. The geochemical data suggest that did not formed by simple dehydration melting of basaltic rocks or fractional crystallization of basaltic magmas. The features of slab-derived fluids expected from recent high pressure experimental studies indicates that mantle wedge is partly metasomatized with “rhyolitic” materials from subducted slabs; it is more likely that very low degree partial melting of the metasomatized mantle wedge formed the rhyolite magma.

Paleozoic U-Pb age of rutile inclusions in diamonds of the V–VII variety from placers of the northeast Siberian platform

1151 Diamonds attributed according to the classifica tion of Yu.L. Orlov to the V and VII varieties are wide spread in rich placers of the northeastern part of the Siberian Platform [6]. Mineralogical investigation (certification) of these diamonds showed that they represent a unified genetical type subdivided by Orlov according to the following formal characteristics: dia monds of the V variety are represented by monocrys tals, and diamonds of the VII variety are represented by intergrowths. Thus, we unite them into one variety V–VII [2]. Diamonds of this joint variety are easily distinguished among other diamonds from placers of the northeastern part of the Siberian platform for the following reasons: (1) these diamonds are represented by crystals of octahedral habitus (rarely) or their inter growths; (2) they occur in the forms of dissolution of dodecahedroids and intermediate forms between octahedrons and dodecahedroids (often); (3) crystals are overfilled with black flaky inclusions (graphite along the walls of flattened vacuoles with fluids; (4) the surface of crystals is covered by scars and fissures of leaching along the vacuoles brought out into the open surface during dissolution (Fig. 1a); (5) these dia monds are characterized by increased size (crystals less than 1 mm are practically absent); (6) lightened isotope composition of carbon, δ13С varies from –19 to –25‰, which corresponds to the characteristics of crustal carbon of organic origin established for a sig nificant part of diamonds from eclogite paragenesis [10]; (7) extremely high structural admixture of nitro gen mostly in the A form (up to 1200 ppm); (8) abundance of fluid inclusions (mostly such compo nents as СО2, Н2О, N); (9) reduced density (3.500– 3.508 g/cm3, diamonds of the I variety show density values about 3.51543 g/cm3). Eclogite paragenesis of these diamonds is proved by occurrences of coesite inclusions [7]. Such diamonds are absent in the known kimberlites and lamproites of different ages in the Siberian platform and worldwide.

U–Pb age determination for zircons using laser ablation-ICP-mass spectrometry equipped with six multiple-ion counting detectors

Precise zircon U–Pb age determinations have been made on Plesovice zircon using laser ablation-multiple ion counting-inductively coupled plasma-mass spectrometry (LA-MIC-ICP-MS). To achieve high precision and high spatial resolution age determination, multiple ion counting using six electron multipliers was employed. The intensities of Hg–Pb–U isotope (202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, and 238U) signals were monitored simultaneously without mass scanning. In static acquisition mode, the resultant 238U–206Pb concordia age for Plesovice was 336.3 ± 1.9 Ma, demonstrating improved precision over that achieved using a magnetic sector-based single-collector-ICP-MS, which was 340.3 ± 3.5 Ma for Plesovice. A high duty cycle can be achieved, along with a short integration time or a small sample volume for analysis, allowing high spatial resolution. More importantly, downhole fractionation can be reduced with a shallow ablation pit. To take full advantage of the setup, a one-second LA analysis (8 laser shots with an 8 Hz repetition rate) was adopted for U–Pb age determination. The resultant concordia age for Plesovice was 339.5 ± 6.7 Ma, demonstrating that the repeatability and laboratory bias precision of the resultant age data were comparable to conventional ablation with a single-collector-ICP-MS. The depths and crater diameters of the ablation pits were, respectively, about >1 μm and 25 μm. The data presented herein demonstrate clearly that multiple ion counting-ICP-MS can become a fast and user-friendly tool for use in U–Pb zircon geochronology.

Evidence of a modern deep water magmatic hydrothermal system in the Canary Basin (eastern central Atlantic Ocean)

New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.

Mantle wedge deformation by subducting and rotating slab and its possible implication

We have constructed a simple model of the deformation of the mantle wedge caused by the subducting and rotating slab based on corner flow model. By applying this model to the geological settings of the Somuncura plateau volcanic region, northern Patagonia, which is located far from the volcanic front, we constrain the mechanical aspect of the hypothesis that the volcanisms of the Somuncura region are triggered by the dehydration-induced melting of the up-warped transition zone which may contain more water than other regions do. Assuming that the water concentrates in the transition zone under the Somuncura plateau, we find that the above scenario may be possible, if the speed of the subducting slab is less than ∼2 cm/yr or the dip angle changes significantly (∼50 degrees within ∼8 Myr).

The tectonic setting of the eastern margin of the Sino-Korean Block inferred from detrital zircon U–Pb age and Nd isotope composition of the Pyeongan Supergroup (upper Palaeozoic – Lower Triassic), Korea

The upper Palaeozoic succession (Pyeongan Supergroup) in central eastern Korea is well correlated with the equivalent successions distributed in North China, suggestive of the Korean upper Palaeozoic being part of the Sino-Korean Block. Detrital zircon U–Pb ages and Sm–Nd isotope compositions of the Pyeongan Supergroup in the Samcheok coalfield of the Taebaeksan Basin were analysed. A single predominant zircon age peak at c. 1.9 Ga (> 70%) is marked in all sedimentary units, followed by varying amounts of minor late Palaeozoic grains (up to 30%). The rarity of Meso- to Neoproterozoic- and Silurian-aged zircons confirms that sediment influx from the South China and Qinling blocks was insignificant. The 2.0–1.8 Ga-dominated zircon age pattern and the Nd isotope composition (average eNd(0) = −15.5±4.0) of the Pyeongan Supergroup most closely reflect the signature of the Yeongnam Massif basements, which supports a previous hypothesis that the Pyeongan Supergroup was mostly derived from a palaeo-orogen located to the east–southeast. Relatively higher eNd(0) values (> −10.1) in the lowermost and the upper parts of the succession are closely matched by the increased occurrence of syn-depositional-aged zircons, which indicates considerable mixing of juvenile materials at c. 320 Ma and 260 Ma. Both arc-related magmatic events are interpreted to have been related to oceanic subduction, suggesting that the eastern margin of the Sino-Korean Block was an active continental margin during late Palaeozoic times.

Corrigendum Motoki et al, 2006 (vol. 33 n1)

A new shallow multichannel seismic survey was carried out in the Llancanelo Lake region (Southern Mendoza Province, Argentina) in order to depict the major Neogene sedimentary-volcanic sequences and regional evolution of a tectonic-volcanic basin located in a key region at the eastern foot of the Andes cordillera. This research is settled on early studies that comprised seismic works reaching depths of 600/700 m as well as geoelectric and electromagnetic surveys reaching the uppermost 80-100 m of the sequences. The processing of the reflection seismic data enabled to obtain a stacked section useful for a preliminary geological interpretation. The obtained results indicated the presence of three major sedimentary units with increasing volcanic (basaltic layers) intercalations with depth that accommodate to the geometry of the depocenter. The entire sequence encompasses most of the Neogene. The previous research set the methodological basis for performing more detailed shallow seismic surveys and allowed defining the strategies for obtaining a better constrained geological model. Seismic transects located north of the lake were densified by adding new profiles. Deep seismic and borehole data from the industry were included in this study in order to provide a regional stratigraphic structure of the Cenozoic sequences and to contribute to the knowledge of the final stages of evolution of the Llancanelo basin.The Cuyania stratigraphic record of the Pygodus serra Zone, and the E. robustus and E. lindstroemi subzones, is discussed in this contribution. Three classical sections have been sampled for conodonts in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The Ponon Trehue and La Cantera formations start with conglomerates and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section, exhibits a massive sandstone bed with carbonate nodules between the black shale. The key conodonts recovered indicate a late Darriwillian age for the bearing strata, these conodonts specimens from the Ponon Trehue region are compare with those from the Precordillera. Finally, the studied clastic sedimentary successions that record this biozone have been compared, concluding that eustacy did not control the sedimentary change in Cuyania for this time interval.In this contribution we present new specimens of Litopterna recovered during the last decade in Quebrada Fiera (Mendoza Province, Argentina), whose fossiliferous sediments, currently recognized as the base of Agua de la Piedra Formation, are assignable to Late Oligocene (Deseadan South American Land Mammal Age). Two remains mentioned in the first publication on this locality were neither detailed nor described, and they have not been located in the corresponding repository. The new material consists of postcranial fragmentary remains (astragali, calcaneum, and metapodials) of Macraucheniidae (Cramaucheniinae) and an incomplete upper molar (M3) of Proterotheriidae (Proterotheriinae). These few remains of litopterns contrast with the abundance of notoungulates at Quebrada Fiera. A comparative study was carried out with material from Patagonia (Argentina) and taxa recorded in Bolivia and Peru for the same temporal interval. The specimens of Cramaucheniinae are assigned to Coniopternium andinum and the molar of Proterotheriinae to cf. Lambdaconus suinus. This contribution allows us to extend the geographical range of Coniopternium, filling the gap between the Patagonian and lower latitude localities (Bolivia and Peru) in which this genus was found. The record of L. suinus in Quebrada Fiera expands the geographical range of this species outside from Patagonia.ABSTRACT. The aquatic sloth, Thalassocnus, is one of the most intriguing linage of mammal knew from the southern pacific coast of South America during the late Neogene. It was initially recognized in Peru, but recent paleontological surveys also demonstrate its presence in Chile. However, the determination of how many species of Thalassocnus were in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (7-7.5 Ma), Bahia Inglesa Formation (Atacama Region, northern Chile), which shows affinities with Thalassocnus. Morphological comparisons with others forms from Chile and Peru allow us to attribute the CPUC/C/557 specimen to T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahia Inglesa, Horcon and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and a younger species than T. natans (probably T. yaucensis). Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of this extraordinary and enigmatic clade of organisms.The uppermost Cretaceous continental deposits of the Neuquen Basin (west-central Argentina) have yielded a high diversity vertebrate assemblage, including numerous dinosaur species. The Neuquen Group is the richest dinosaur-bearing unit of the basin and comprises the Rio Limay, the Rio Neuquen and the Rio Colorado subgroups. In this group, there is abundance of isolated, disarticulated or partially articulated sauropods and theropods. However, little is known about the taphonomic history of fossil assemblages. In this study, dinosaur remains from the Rio Neuquen Subgroup (Plottier Formation) found in the Cerro Guillermo area in southern Mendoza are examined. The investigation of fossil occurrences within the study area revealed the existence of different taphonomic modes, from isolated bones until partially articulated skeletons preserved in floodplain and channel settings. SEM-EDS analysis showed the substitution of hydroxyapatite by francolite in the bone microstructure. The presence of fluorine –in one of the cases– suggested a link between the elemental composition and depositional environments: floodplain and fluvial channel. The survey of the vertebrate accumulation types and their sedimentary context allowed documenting a wide range of processes responsible for their genesis, operating within a fluvial-dominated environment. This contribution represents an holistic approach about taphonomic history of Cretaceous dinosaurs for assessing the differential preservation of fossil assemblages in fluvial environments.

Ages of zircons from Jurassic sediments of Bluefish River slope, NWT Canada, and the possible age of kimberlite activity on the Lena West property

This work is based on a study of 30 zircons extracted from the heavy mineral fraction of a sedimentary rock sample with anomalously high contents of well-preserved kimberlite indicator minerals (KIM). The sample was taken from the basal horizon of Jurassic‐Cretaceous sandstones on the slope of Bluefish River valley. The Bluefish River, located in the Northwest Territories (Canada) north of Great Bear Lake, is the right tributary of the Mackenzie River. The Bluefish River basin is a part of the large Lena West property that occupies an area of about 40 000 km 2 . Based on available tectonic data [1, 2], the area is situated within the Paleoproterozoic Hottah terrane and is separated from the Slave Craton to the east by Wopmay orogen. The Lena West property represents the sedimentary basin of a stable platform, which is filled with a thick sequence of mainly continental Cambrian‐Cretaceous sediments. The Bluefish area is dominated by undivided Jurassic‐Cretaceous rocks (clay shales, siltstones, and sandstones), which make up the Parsons Series and represent terrigenous sediments of the continental margin. Undivided Cambrian‐Devonian rocks are exposed only at the lower reaches and in the immediate vicinity of the river valley. Glacial deposits are subordinate. During prospecting of this area for diamonds, anomalously high contents of KIMs (primarily, Cr-pyrope and picroilmenite) were found in the alluvial samples of the middle reaches of the Bluefish River. Sedimentary rocks were sampled on the slope in detail (Fig. 1). The highest KIM contents (hundreds of grains) were found in samples from the basal horizon of Jurassic‐ Cretaceous sandstones. The Jurassic basal horizon rests on the limonitized gray siltstones of the Devonian Hare Indian Formation. The basal horizon (about 30 cm thick) includes three lithologically diverse interbeds: quartz gravelstones (upper interbed), limonitized coarse-to medium-grained sandstones (middle interbed), and yellow-green clays with a small admixture of pebbly material (lower interbed).